Hi, small deviations are conceivable and I also had them, but they were about 200-300m on my home course of 12km.
I think 1.4km is rather a lot, even for a marathon.
If necessary, check the Garmin profile to see where the deviation comes from.
Have you compared it with the reference (measurement)?
I would also check the recording frequency.
It may be that it needs to be readjusted with a new belt...